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Would mid-air refueling for an air-to-space vehicle that flies like a plane be a viable advantage if the tech was available?
Let's take the style of an F-22 Raptor (awesome!), build the fuselage, wings, etc to handle runway takeoff horizontally and then vertically (if needed) up to orbital space flight to reach the ISS in order to deliver fresh StarBucks coffee, then back to earth, all this thanks to jet-rocket engines that can handle the fuel needed for atmospheric and space flying while using one main fuel tank. In this scenario, considering what we already have as tech or in near future, would mid-air refueling with rocket fuel be a better alternative than having to piggy-back on 1rst stage rocket boosters or ride atop a stratospheric plane for launch in upper atmosphere?
I made a video of this fictitious exploit in 2010, and would like some constructive feedback concerning the refueling part and any other details worth debating related to the question that seeks to allow orbital flight with SSTO space vehicles using runways like planes thanks to the refueling idea (which means less fuel to carry upon runway departure).
Latest UPDATE
For those who have no clue what I am talking about or in order to avoid answers and comments that do not rely on the question's reliance to "...if tech was available?", please visit SABRE (rocket engine) for more on the topic of current hybrid rocket engine dev which is a source of inspiration.
Last UPDATE (May 6, 2021)
The "answer" from "HiddenWindshield" does not relate to my question. The question is closed and no more answers are being accepted. Thanks in particular to Guy Inchbald who has provided essential data for pertinent analysis and review.

< would like some constructive feedback concerning the refueling part and any other details worth debating> that's not how this website works. We do not debate, we don't provide opinion, we don't guess about what "would be a viable advantage", we strive to provide factual answer to practical aviation questions.

Comments are not for extended discussion; this conversation has been moved to chat.

As you've already learned from the comments, the hard part of getting to space isn't the height, it's the speed. The Saturn V broke the sound barrier less than 30 seconds after launch. So, how is your refueling plane even going to keep up? The only possible answer, at least with modern technology, is to make it a rocket itself.
So, now you've got two rockets that need to rendezvous at supersonic or even hypersonic speed in order to transfer fuel. Not exactly the easiest maneuver to accomplish. Even military pilots that train for hours and hours to do this need calm air, a steady hand, and lots of patience to accomplish this. Transferring fuel at supersonic speed is not going to be in any way practical or safe, even allowing for computer assistance. Much easier to connect the two craft on the ground, and let them fly in tandem until one has been drained of fuel and can then separate.
Now let's talk about the "Tyranny of the Rocket Equation". In short, fuel has mass, so you need to burn fuel in order to loft the fuel that you need to burn to loft your payload. Using current chemical rockets, your refueling craft would need to carry three to six times as much fuel as the orbiter does, just so that it will have enough left over at the end of its flight to refill the orbiter. This of course means that it will need engines that are more powerful as well, to carry all that weight.
In fact, if you upgrade the fueler's thrust just a little bit more, it could carry the orbiter's weight as well. That would even save a little mass (and cost), since now you don't have to have transfer pumps and plumbing to refill the orbiter, as its engines won't even start until the fueler detaches. Wait, hasn't someone else already had this idea?

Comments are not for extended discussion; this conversation has been moved to chat.


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Drag estimation of Xfoil and Vortex Lattice Method?
I have been working on building a software to estimate lift and drag properties of a fixed wing being tested at a wind tunnel. The setup is a low AR wing with NACA0012 profile and 0.3m chord, with 0.4 span lengths being tested at speeds up to 20m/s. For modeling, I'm using Vortex Lattice Method (VLM) for estimating $C_L$ and $C_{Di}$ of the wing. In addition to this, I am planning to use Xfoil's predictions for estimating $C_{Dp}$. I am aware this is a crude estimation, but my starting point, nevertheless. With this configuration in mind, I've been running a few tests to validate the modeling behavior; but there have been some observations that I cannot wrap my head around:

For drag, I have the following plot as my baseline expected behavior. However, XFoil estimates show an opposite behavior for parasitic drag ($C_{Dp}$). When I change the Re from 250,000 to 500,000 (doubling the speed) (at 12$^{\circ}$AoA), $C_{Dp}$ goes down from 0.038 to 0.026. I am aware the drag force is still greater when $C_{Dp}$ is multiplied by dynamic pressure, but decrease in $C_{Dp}$ seems counterintuitive to me.

Continuing off from the figure above, I expected induced drag ($C_{Di}$) to significantly decrease as speed increased. However, both of the VLM packages I experimented with (PyVLM and AeroSandbox) give me constant ($C_{Di}$) at varying speeds. Is this a huge inaccuracy of VLM or am I interpreting something wrong?

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What is the physical explanation for the pre-stall judder in a Spitfire?
From the book "First Light" by Geoffrey Wellum, where the author describes dog-fight manoeuvres against an Me 109:

If you want to shake someone off your tail you have to fly your
Spitfire to its limits. In a tight turn you increase the G loading to
such an extent that the wings can no longer support the weight and the
plane stalls, with momentary loss of control. However, in a Spitfire,
just before the stall, the whole aircraft judders, it’s a stall
warning, if you like. With practice and experience you can hold the
plane on this judder in a very tight turn. You never actually stall
the aircraft and you don’t need to struggle to regain control because
you never lose it. A 109 can’t stay with you.

What is the physical explanation for the pre-stall judder? Is it some sort of unstable linear-turbulent flow transition across the wings, or something else?

Flying to the buffet is a common method of achieving a maximum-rate turn in an aircraft where the g limit is high enough to allow it. Select maximum power to enable the highest g loading, roll, and then pull progressively until you feel the whole aircraft buffeting, then hold it. You can go even tighter if you aren’t concerned about maintaining your level by rolling to 90°.

The 109 was a pig to fly. About 30% of the 109s lost in WWII were not shot down in combat, but crashed on takeoff or landing.

It wasn't a pig to fly at all.  The slats allowed you to pull very high G without it departing in a snap roll if you went past the limit.  It was a pig to take off and land being ground loop prone.  In spite of that, many German pilots preferred it to the FW190, because experienced pilots could handle the exciting ground handling.

You thermal a Schweitzer 1-26 glider exactly like this.  The optimal min sink condition is achieved with stall buffet starting, and the buffet was quite strong, so you go around with the thing shuddering all the way.

@Arkhem: It's also quite common in GA training planes, or at least the ones I've flown (C-150 and PA-28-180, mostly).  In training, I would have to fly just to the edge of stall, where the buffeting starts, and then maintain that speed & attitude.

@alephzero High losses on takeoff and landing is also a symptom of having inexperienced pilots. Even today, Boeing research suggests nearly half of fatal accidents happen on approach and landing, and 15% during takeoff. At the start of the war, Germany had plenty of pilots, and it was the Allies who had to scramble to train more, but later in the war, Germany had lost many of its experienced pilots and was hampered in its efforts to train more by the loss of aircraft, airfields, aircraft factories, and fuel inflicted by the Allied bombing, not to mention the dwindling areas of safe airspace.

The Spitfire wing had this behaviour designed in. Mitchell's aerodynamicist Beverley Shenstone had a good deal to do with it. I can't remember who was the main design lead on the wing, but it was not Mitchell himself.
A constant-chord wing will experience some burble before the stall, due to buffeting as the airflow begins to separate near the trailing edge. It was well known that on a tapered wing with no tip washout (reduced AoA), the tips stall first and very suddenly. Worse, the stall then made the ailerons ineffective. In order to avoid this, the Spitfire was given 2.5 degrees of physical twist or washout. The tip aerofoil camber was also reduced (sometimes referred to as aerodynamic washout), to further reduce loading and lower its stall speed.
As a result, the inboard section stalled first, complete with warning burble, while the ailerons remained effective.
The Spit also had a much longer mean chord and hence lower wing loading than the Bf 109, which also lowered its stalling speed.
Although the Bf 109 wing had leading-edge slats to increase tip lift and delay the stall, it had a shorter chord and higher loading; the combination of features on the Spit still allowed it to out-turn the Messerschmitt.

Very interesting. Earlier in the book, it is mentioned that the Harvard training aircraft had very sudden stalls which needed some care to recover from, was that a result of the wing design?

@DrMcCleod Don't know much about the Texan/Harvard, but a nasty stall was still common in the mid-thirties and quite likely expected, even if not an actual design requirement for an advanced trainer.

The 109 had outer slats which prevented outer stall at any reasonable AOA and  so could do pretty much the same thing.  Your last sentence is the real answer.  The difference was the wing loading.

@JohnK Really, it was the combination of the two. A Spit lacking washout would also have stalled at a higher speed. I updated my answer accordingly

It's because aircraft don't stall all at once.
A stall occurs when the airflow over the upper surface of the wing separates from the entire upper surface, causing loss of lift.  This airflow separation starts at the trailing edge (some separation right at the trailing edge often occurs even in normal flight).  As the aircraft's angle of attack increases, the point on the wing where the airflow separates from the upper surface moves further and further forwards.
Up to a point, the lift generated by the wing continues to increase with increasing angle of attack, despite the growing separation bubble, as the wing deflects air more and more downwards.  Past a certain critical angle, though, the forward edge of the area of separated flow moves forwards very rapidly until it's nearly at the leading edge, lift drops off, drag increases sharply, and the wing's stalled.
The separated airflow is highly turbulent, and, once the pocket of separated airflow spreads far enough forwards and becomes big enough, this turbulence can sometimes be felt by the pilot as buffeting or juddering as the large amounts of turbulent air flow rearwards and strike the aircraft's horizontal tail.  Whether this can be felt before the aircraft actually stalls depends on the aircraft:

Many aircraft, including (apparently) the Spitfire, have fairly-docile stall behaviour, with the separated flow producing noticeable buffet over a considerable angle-of-attack range before the wing finally stalls.  These are mostly aircraft built with at least one of the following:1

Thick wings, which cause the area of separated flow to grow slowly and gradually rather than all-at-once.
Low-mounted horizontal tails, which allow the turbulent airflow coming off the wings in the area of separated flow to strike, and buffet, the horizontal stabilisers and elevators.
Washout (a slight twist built into the wing going from root to tip, so that the angle of incidence - and, thus, the angle of attack - at the wing root is slightly higher than at the wingtip), which causes the airflow over the wing roots (the portion of the wing whence comes the turbulent airflow responsible for striking the horizontal tail and producing pre-stall buffet) to separate earlier than that over the wingtips,2 which provides a warning (in the form of said pre-stall buffet) earlier than would otherwise be the case.3

Other aircraft (generally those with fairly-thin wings, high tails, and little-to-no washout, such as - rather infamously - first-generation Learjets) stall abruptly, with the area of separated flow growing almost instantly from too-small-to-noticeably-buffet-the-tail to encompassing-the-entire-upper-surface (i.e., stalled).  Since there's no significant warning buffet before the actual stall, these aircraft need stickshakers to warn the pilot of an impending stall.

1: In the specific case of the Spitfire, the washout was the deciding factor (thanx @Mark and @Guy Inchbald).
2: This is especially important for aircraft with rearward-swept wings (where, in the absence of washout, the effects of spanwise airflow along the wing cause the wingtips to stall at a lower angle of attack than the wing roots, causing the aircraft to pitch up violently and be difficult to control in roll at high angles of attack), but is beneficial even on straight-winged aircraft like the Spitfire.
3: Another benefit of wing washout is that the lower angle of attack at the wingtips - where the aircraft's ailerons are mounted - enhances the ailerons' control authority (ailerons become less and less effective with increasing angle of attack), preserving roll control at high angles of attack.

Good answer, but a separation bubble happens in laminar flow, followed by transition and reattachment. Here you talk of turbulent flow separating without a chance of reattachment, so there is no bubble involved. Also, what you feel is mostly that separated, highly turbulent flow hitting the tail. High-frequency lift variations on the tail cause that judder. The T-tail of the Learjet is too high to be hit by separation turbulence; therefore it needs the stick shaker.

Separation bubbles that trigger leading edge stall that cascades down the wing all at once require a pusher.

Back-to-front stall might be the cause of buffet in the general case, but in the specific case of the Spitfire, the buffet comes from the wings being designed to stall from the root outwards.

Note that the Spit has an unusually thin wing for the day, at 13.5% T/C, and a tail a little above the wing. Washout was the key.

The reason for this is a phenomenon called buffeting. The initial separation of the boundary layer induces vibrations in the wing (which at certain speeds can cause flutter). In most aircraft the stall warning in the cockpit is set off by buffeting.

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Can airplanes benefit from this "roughness" on bird wings?
Why do birds have so much rough surface (overlapping feathers) compared to aircraft? Isn’t that cause additional skin friction drag?
Could planes maybe benefit from this?
link:
https://www.researchgate.net/publication/263394314_Gliding_Swifts_Attain_Laminar_Flow_over_Rough_Wings

It may seem counterintuitive, but by introducing a small amount of turbulence, this interrupts the slower boundary layer which is directly in contact with the wing, thereby delaying local airflow separation and aerodynamic stall. Essentially, at the expense of a small amount of parasitic drag, the stall performance and control effectiveness of the wing can be improved.
Aircraft add small fins called Vortex Generators to achieve this effect on otherwise smooth wings, flaps, ailerons, rudders, and elevators.

Here is a link to check out:
https://en.m.wikipedia.org/wiki/Vortex_generator

Also, the Reynolds number of birds and airplanes are very different. What works for birds may not be necessary or even helpful at other Reynolds regimes. What works for insects also doesn't work quite as well for birds.

@JZYL you mean because speed of airplane is so much faster then birds?

Please google Reynolds number.

@AeronauticFreek Because of size and speed

when you say size, you mean because birds chord is so much smaller compare to airplane chord?

Another factor is that bird wings (and to some extent the feathers) have a highly variable geometry that's under very precise control.  The bird can thus position the wings to minimize drag (or whatever) as its speed changes.

@AeronauticFreek reynolds number just tells us how (un)important the viscosity of the air is relative to the plane. And yes, it increases with both chord and speed. For larger, faster planes, the viscosity of the air is less important (though it still is very important).


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How did the 9/11 flights recieve cell phone signals?

The maximum distance at which a phone can still make calls and send texts varies depending on the type of tower and transmitter, but an airplane would have to be no more than 10,000 feet in the air for any cell phones on board to still have a signal, according to Bill Rojas, director of telecom research at IDC Asia Pacific.

https://www.cnn.com/2014/04/14/tech/mobile/phones-in-flight/index.html#:~:text=Despite%20the%20urgings%20of%20flight,of%20feet%20into%20the%20air.
How were passengers able to make cell phone calls? Did all of them use airphones?

Apart from the fact they happened to be on an aeroplane, this question has nothing to do with aviation.

@Jamiec is asking about recsrpyion on planes offtopic

Yes, it is. See https://aviation.stackexchange.com/help/on-topic - at best its about The "passenger aspects" of commercial air travel, but really its a question about the physics of radio waves

No, I believe very few of them used satellite phones.
An article on the New York Times from 14-09-2001 states:

According to industry experts, it is possible to use cell phones with varying success during the ascent and descent of commercial airline flights, although the difficulty of maintaining a signal appears to increase as planes gain altitude. Some older phones, which have stronger transmitters and operate on analog networks, can be used at a maximum altitude of 10 miles, while phones on newer digital systems can work at altitudes of 5 to 6 miles. A typical airline cruising altitude would be 35,000 feet, or about 6.6 miles.

Now, this theory was valid then (during 9/11) but not for modern aircraft like the A320, 737s, etc. Why is that so?

Jon Brittingham, a technical pilot in the Airbus A319/320/321 program,
explained the causes to me in greater detail. Older aircraft, such as
the McDonnell Douglas MD-90, don’t have the same electronic-systems
shielding to protect cabin equipment from third-party signals that
more modern airliners, such as the current Airbuses, have. In fact,
this shielding is most dense around the front of the cabin—thus
confirming my pilot friends’ agreement about poor reception within the
cockpit. (This same fact can also cause cell reception in first-class
seats to be the worst in the plane, when parked by the gate.)

In modern day aircraft, signal reception is poor even when on ground (but inside the aircraft), it becomes worse on takeoff and negligible while climbing and on cruise.
According to me two factors come in hand, Older cellphones had stronger signals, previous generation aircraft (like the B757 in 9/11) might have lacked proper shielding to prevent 3rd party EM signals.
You can refer to the following articles:

https://www.nytimes.com/2001/09/14/us/after-attacks-communications-new-perspective-issue-cell-phone-use-planes.html
https://www.theatlantic.com/technology/archive/2019/12/why-cell-reception-so-bad-after-boarding-plane/602812/

 Fun Fact:
I've observed mobile phone signals are terrible and mostly do not work on boarding a 787. It turns out that the 787, while fabricated from composite materials, uses a wire rebar for structural integrity.  That rebar apparently interferes with cellular signals on the plane.


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Is there an advantage to combining trim function with yoke?
SUMMARY: What if the yoke didn't require constant pressure, as is the case with the current "trim" function?
In all cases that I know of, airplanes have had the yoke (which you need to apply constant pressure on) and trim (which you don't) separate. The yoke is used to short inputs, such as a roll or a pitch down, and the trim is used to maintain the aircraft in a specific attitude, given the changing nature of center of lift and gravity.
But I do not understand why it is necessary for them to be separate. Trim can do (almost) everything a yoke can. Would it be a good idea to combine the two?
Some advantages:

simplification
Safety; fast reaction to sudden trim changes (such as part of wing or empennage breaking off)

Some drawbacks:

trim does not center itself, so there's that inconvenience
increase chance of wild maneuvers if left unattended while deflected

"which you need to apply constant pressure on" ummmm, no. Oh the golden days when the instructor noticed I had not trimmed the plane to zero stick force. Instant slap on the wrist followed.

@MichaelHall tried a fix. better now?

@MichaelHall ah, well, I don't know everything :D. I'm editing agin

Note that Airbus FBW aircraft have an Autotrim function that does pretty much the same as what you’re asking about.

The trim can't really do anything the yoke can do.  The yoke controls the elevator while the trim wheel controls the much smaller trim tab.  Flying by using the trim tab is very inefficient

I think you misunderstand what the purpose of trim is, which is to change the attitude of static stability.
The yoke (or stick) is the primary flight control. Virtually all changes in attitude will originate here, and any temporary changes also end here. If you take your hand off, the plane returns to the original attitude.
When you want to make a permanent change, you set the new attitude using the yoke (or stick) and then trim until the required pressure goes away. When you take your hand off the yoke (or stick), the plane remains in the new attitude.
Some planes do have an elevator trim control on the yoke (or stick) to make things easier on the pilot, but this doesn’t change how it is used. Some planes also have rudder and aileron trim controls; I’ve never seen those on a yoke (or stick), but I’m sure someone, somewhere has tried it.

The advantage of NOT having the trim controls on the yoke is that its far simpler, mechanically. For instance the elevator trim on a Piper Cherokee is basically just a wheel attached to a cable running straight back to the trim tab. Imagine the Rube Goldberg design that'd be needed to put it on both control yokes. (Of course if you have more complex "fly by wire" implementations, as in airliners, it's pretty easy to put the control button wherever you like.)


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What made Concorde so heavy?
The Concorde burnt five times more fuel per passenger mile than the 747. A huge contributor must have been the immense empty weight per seat, three times that of the B747.

B747-300
Concorde

Empty Weight
174,000 kg
78,700 kg

Passengers
400–660
92–128

EW/Passenger
263–435 kg
615–855 kg

This weight excess is distributed among the slender fuselage, thin wings, long landing gear, big engines, and (possibly) material constraints.
But how much of the weight increase comes from each of these components, and why? I am looking for a comparison with the weight breakdown of a conventional jetliner

The Concorde mainly transported fuel across the Atlantic so that enough was available to land safely. Passengers were just an extra on top. Also, being first-class only, it held fewer passengers than what low budget airlines have taken to cramming into their planes lately.
To look at the structure relative to passengers is a bit misleading. Let's look at the fuel load and the mass fraction of empty weight relative to MTOW instead:

747-100
Concorde

MTOW
333.4 tons
186.88 tons

Empty mass
162.5 tons
78.9 tons

Fuel
183,380 liters
119,800 liters

Fuel to empty mass
1.1285 L/kg
1.5146 L/kg

Mass fraction of empty to MTOW
48.74%
42.22%

If you compare passengers plus fuel to the empty mass of each airplane, the Concorde actually comes out well ahead of the 747 and looks like a prize of lightweight design. Having structurally efficient delta wings helped and should explain most of the difference.

Also, could you give that fuel fail in numbers?

@Abdullah What is a fuel fail?

Fuel fail is a portmanteau of "fuel" and "epic fail". It is used to describe a situation where a passenger aircraft becomes a fuel tanker, and the customer of this fuel tanker is the same aircraft, and this fuel is needed to ensure that the aircraft lands in such a way that leftover fuel is not spilled.

I was referring to that thing about transporting fuel over the ocean just to land. Sorry for the lazy wording

@Abdullah: Carrying fuel for cost difference reason is also known as tankering.

A major factor would seem to be just the square-cube law.  The material needed to make a tin can increases as the square of its diameter, but the amount of stuff you can put in it - whether sardines or humans - increases as the cube.  Doing the same calculation for e.g. a Citation X (12 passengers) gives about 820 kg empty weight per passenger.  Then the Concorde has to be stronger and have more fuel tankage to cope with supersonic flight...

@ymb1 Put four afterburning engines on an MD80, plus fuel to feed them pushing the plane to supersonic speeds, plus wings large enough to hold all that fuel, and weight will probably approach Concorde’s.

The square-cube law makes large aircraft less efficient, not more. The surface grows slower, but you need to make the structure stronger to hold the weight and the wings must grow faster to produce enough lift and be stronger to carry it and that tips the balance the other way.

@ymb1: From a quick search, the MD-80 fuselage seems to be rather larger in diameter than the Concorde: 3.34 m vs 2.88 x 3.32 m.  https://en.wikipedia.org/wiki/McDonnell_Douglas_MD-80#Specifications http://www.concordesst.com/dimentions.html


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Does increased fineness ratio decrease skin friction drag?
From the Wikipedia article on parasitic drag:
a designer can consider the fineness ratio, which is the length of the aircraft divided by its diameter at the widest point (L/D). It is mostly kept 6:1 for subsonic flows. Increase in length increases Reynolds number. With Reynolds no. in the denominator for skin friction coefficient's relation, as its value is increased (in laminar range), total friction drag is reduced.
The higher fineness ratio gives a longer flowpath, slowing the air more (=higher Reynolds). A slower boundary layer creates less friction downstream.
But increasing the fineness ratio increases the total surface area as well. And that is a recipe for increased friction. From the Wikipedia article on fineness ratio:
As the name implies, this is drag caused by the interaction of the airflow with the aircraft's skin. To minimize this drag, the aircraft should be designed to minimize the exposed skin area, or "wetted surface"
So, what am I missing?

Done, though these quotes are from the current article.

Basically, your confusion comes from not knowing what needs to be held constant.  When you increase the fineness ratio without increasing wetted-area, i.e. you hold wetted-area constant, the overall drag goes down.  You can see this in racing shells (sculling).  The shells (boats) have an absurd fineness ratio, very narrow and very long.  This decreases the overall drag (somewhat, there are other forces at work there).
As you increase the finess ratio, at the expense of increasing the wetted area, you will need to do a trade-study to determine if you gain an advantage.  You probably will, up to a fineness ratio of 6:1.  Much beyond that the extra drag from the increase in wetted-area starts to dominate any savings in drag due to body fineness.
The inital statement is true, presuming you keep the wetted-area constant as you adjust the fineness ratio.
T

then the volume decreases.


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Can water-cooled brakes be used for takeoff aborts?
Airliner engines are sized to continue the takeoff if one engine fails after it has become too late to brake. This means that the oversized engine has to be lugged around the whole flight, increasing weight and drag. Creating traditional brakes that could withstand the heating of a rejected takeoff just before rotation would also add significant weight.
The brakes on the SKYLON spaceplane boil water during abort, and it appears that the steam is dumped. The water is fed by pyrotechnic blowdown. Following a successful takeoff, the water is dumped overboard. This saves weight for the remainder of the flight.
My question is, can this approach be used to save weight on airliners?
SKYLON info.
https://web.archive.org/web/20151129034506/http://www.reactionengines.co.uk/tech_docs/SKYLON_Users_Manual_Rev_2.1.pdf
https://web.archive.org/web/20110615104534/http://www.reactionengines.co.uk/tech_docs/JBIS_v57_22-32.pdf
https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=36826.0;attach=1073534

How does water cooling solve the primary braking problem, friction (or lack thereof) between tires and runway?

it's about temperature

@jamesqf have you ever seen footage of an emergency brake test on an airliner? Typically, by the time the plane stops the brakes are on fire and the tires have exploded.

I wonder what this does to the brakes. If it does more damage than overheating the brakes, we might as well let the brakes overheat. I'd say it's an OK deal to let the brake burn its entire service life by one RTO.

But a fire.........?

Watercooling a set of brakes is not an issue. This is used routinely in truck racing. You'd have an issue if you hose down a set of hot brakes (excessively rapid cooling, maybe even quenching) but if you apply water before the brakes heat up you're ok.

See new details above.

A good answer may compare this system to already implemented mechanisms such as air cooling.

This solves the problem of rejected takeoff, which is certainly a major factor in brake design. However, dumping the water after takeoff means that benefit is unavailable for landing. Events such as flap issues can require an aircraft to land at a higher speed than usual, requiring more braking capacity. The difference between this and rejected takeoff may not be enough to justify the weight, complexity, and expense of the water cooling system.

A flap issue could surely allow the full runway to be used, as opposed to a high-speed rejected takeoff surely?

@ABJX you have the full runway, but the speed is higher, and kinetic energy increases with the square of the speed.

So the brakes heat up to rejected-takeoff levels anyway?

@ABJX it all depends on the specifics. Often it's at the end of the flight where much of the fuel has been burned, which helps, but it could also happen right after takeoff where the aircraft is still heavier.

I'm sure it could be used, the question is what problem it would solve that would make it worth doing. Dumping water for each take-off means more stuff that could break on the aircraft itself, and more stuff that requires maintenance every now and then.
Having to refill the airliner with water prior to each takeoff would require extra infrastructure on the ground; if water-cooled brakes are critical for operating the aircraft, you probably wouldn't be allowed to use just any water. Distilled water refilled using a documented process would probably be a must, adding time and making the whole thing more expensive to operate.
Airliners routinely operate in cold weather, so the ground supply and aircraft mechanisms have to account for that. No point having efficient brakes if they are frozen solid as a result of having to wait 10-15 minutes extra prior to takeoff. You can of course install heaters, but that adds additional costs and complexity.
All this stuff would mean nothing for anyone traveling with the airline. Compared to airlines using conventionally braked aircraft tickets are likely to be more expensive, driving people to use the cheaper non-water-cooled airlines.

From what I hear, airliner engines are sized to continue takeoff if one engine fails when the plane is moving too fast. This is especially bad for today's twinjets. And that is why I was thinking of a lightweight high-power braking solution to allow braking instead of needing overpowered engines.

More powerful brakes don't allow you to get rid of engine-out redundancy. If the engine fails just one foot off the ground, you can have the most powerful brakes in the world, they won't help you one bit.

Of course, they could technically use water-cooled brakes, but there are downsides. For example, airports don't want water spilt all over the ground just because one airplane had some hot brakes. It's just more efficient to taxi around and wait for the brakes to cool down. Plus, it would be too expensive to install a whole water brake-cooling system, for something that just doesn't happen that often.

So to sum it up,

It interrupts airport operations

It's too expensive

BONUS: The airline manufacturer would have to get permission for the technology

Would it be more expensive then needing additional engine power to continue the takeoff?

@ABJX it sounds like this comment (which you've made on 2 answers) is really the crux of your question. Possibly this is what you should have asked in the first place.

All right, will add to the question.


text:
Can PPL, CPL, and ATPL be completed within a year?
It took me about 8 months to complete my PPL, because I struggled to go solo and I haven't started my CPL training yet due to financial set back.
I've heard about students who completed their PPL training in 2 months.
Is it possible to complete PPL, CPL and Frozen ATPL in 12 months and how do you go about it? And also, are there any cases? The one I've seen is 18 months.
Considering the fact that you have to write CPL exams, ATPL exams, IR, Night Rating, 100 PIC hours and other related tasks.

About the biggest setback in my part of the world was always the weather. When training for my PPL I swear I spent more time looking at the sky from the ground, than actually in the air.

Which country/regulator are you asking about?

Only fact I can guess is he is from South Africa.

My question is based on South African regulations.

Given the hours required for the ATP, the chance of doing that rating alone in under a year seems pretty remote.
Getting the private and commercial certificates done inside of a year is entirely possible, although not cheap. The writtens for the ATP are no big deal, but the hours simply will take a lot of time, as well as the right employment opportunity, to get.
The number of FO's at smaller (regional) airlines who have been flying for several years but who don't yet have their ATP attests to the time required. You can (and generally have to) pay for the flight time to get your Commercial, but paying for all the flight time required for an ATP is simply beyond those of whose last name isn't Gates, Bezos, Trump, or etc.

Do you mean frozen ATPL? This can be easily done in 3 months if you have unlimited money and normal intellectual qualities.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why does a phone's flight mode also disable FM radio?
I had two phones being capable of listening to VHF FM radio (in Germany about 90 to 104 MHz).
However when turning on flight mode, the radio receiver is also disabled, and I wonder why:
Listening to radio does not transmit any electromagnetic waves (more than when playing some game I guess).
I thought it would be some fun listening to the different radio stations while passing countries from above.

What model phones are they?

My guess is you would not get any reception at 30,000 ft anyway so might as well turn it off.

@vasin1987 - especially in a metal tube with small windows.

@HiddenWindshield AFAIK a LG KE960, a Samsung Galaxy II, and a Huawei P10 Lite.

@U.Windl I was going to say that I didn't know of any phones that had built-in VHF receivers, and make sure that you weren't actually using an internet streaming site. But, having looked up the KE960, I can say that I have now heard of a phone with a built in VHF radio.

All broadcast-band radio receivers contain local oscillators which are used in their intermediate frequency (IF) circuits to downconvert the (higher) frequencies they receive. These IF circuits are prohibited by law from radiating more than very small amounts of radio frequency power while operating, but to prevent any possible chance of having the airplane's navigation receivers pick up interference from portable radios, they have to be shut off during flight.
This is a minor problem today, but old-school shipboard radio receivers during WWII were supposedly capable of radiating enough power off their IF stages that submarines could tune in on them and use them as homing beacons to track convoys and then sink them.

I thought there were some existing questions on this, but I didn't find a good match.

Listening to radio does not transmit any electromagnetic waves (more than when playing some game I guess).

That's not correct.  Although the device is intended to be a receiver, the antenna and circuitry can end up leaking power on or near the received bands.  For a well-designed, healthy unit, this should be minimal.  But it's much easier for such transmissions to come from a receiver than from a game unit that has no antenna and no tuning circuitry.
I remember all the US airlines I flew on in the '90s explicitly prohibited operation of radio receivers during any phase of the flight, not just takeoff and landing.  I haven't surveyed, but I think many more these days allow AM/FM receiver operation.  I have no idea if this is because radios are less likely to cause interference, airplanes are less susceptible to such interference, or folks bringing radios on has dropped to levels that it's not worth mentioning.  But United at least still appears to have them on their "do not use" list.  https://www.united.com/ual/en/us/fly/travel/inflight/devices.html
So I'd say that due to the history, it makes sense that a device with "airplane mode" would disable any radio-band receiver circuitry by default.

Actually my personal guess was that the crew cannot check every receiver regarding the frequency range they could operate in, and in principle you could listen to the ATC traffic then (which is not allowed in all countries).


text:
How can these flights operate legally without tail numbers?
At dusk, several unmarked white buses unload a few hundred people and children who board these white unmarked jets.  They leave and return empty within 24 hours. This is a daily occurrence.  Harlingen (HRL) is located 20 miles from the Los Tomates bridge in Los Indios, Texas on the Rio Grande river. These jets do not have tail numbers or any other ID marks on them.  How can these jets operate legally in the US?

Welcome to aviation.SE! Just for information, the minimum size for registration markings is 12 inches, which isn't very big at all. On a large aircraft and/or in a fuzzy photo, they may be difficult to see.

On the photo you posted, the registration number is clearly visible below the last few windows. It is not legible because of the resolution of the photo, but it is clearly there.

I’m voting to close this question because it asks how something that isn't actually happening (flying without a tail #) can happen.

The airline is “iAero Airways” previously known as “Swift Air”. It is also a major contractor for U.S. Immigration and Customs Enforcement, operating many of the agency's deportation flights as well as flights transporting detainees between immigration detention facilities within the United States.
In this photo, the tail number is clearly visible.
iAero Airways

Lol so the OP pic posted was just too pixelated to tell it was an N number.  I thought it was a skin patch.

No US-registered civilian aircraft can operate legally in the USA without an N-number. A better picture will reveal it.
The questions of who these people are, where they are going, and whether or not this whole process is legal is best referred to another forum than this one.

Government aircraft do not need to follow FAA regulations, and I'm guessing that means they don't need tail numbers. https://aviation.stackexchange.com/questions/88239/do-faa-regulations-apply-to-government-aircraft

@DJClayworth Partly; there's an exemption to the registration requirements for military aircraft only (14 CFR 47.3(b)). It's not a general exemption for all government aircraft. But there are plenty of other regulations that government aircraft are indeed exempt from.

"A better picture will reveal it." – It's actually visible in the OP's as well.

@StephenS, will edit. -NN

@randomhead, will edit or delete. -NN

According to https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_00-1_1A.pdf  "Accordingly, most aspects of PAO are not subject to FAA oversight".


text:
Why is the TPA listed on the Chart Supplement (AF/D) sometimes lower than 1,000 AGL?
I'm looking at B19 in Maine, and it recommends a Traffic Pattern Altitude (TPA) of 995 MSL (800 AGL). Wouldn't 1,000 AGL be safer?

I'm sorry, but what is "TPA" in this context?  I don't think you're referring to Tampa International Airport, and beyond that I'm coming up blank for that acronym.  Also, what is B19?  Can you post a screenshot of what you're seeing?  That would be helpful.

Welcome to aviation.SE! We have a couple of closely related questions: here, here. Does one of them help?

@RalphJ "TPA" is "traffic pattern altitude", and B19 is the airport identifier for the Biddeford Municipal Airport in Maine

B19 is close to the Portland class C airspace, which starts at 1500 MSL, so a TPA of 995 MSL provides 500ft of vertical separation.


text:
Are there any "VFR corridors" in the US apart from the one in San Diego?
As far as I can tell, the only VFR Corridor (see AIM 3-5-5(b)) that exists is in San Diego. Is that correct or are there others elsewhere in the US?

There are only so many Bravo airspaces in the US. Why don't you take a look at them, then come back and answer your own question? (That's allowed and encouraged!)

Take a look at LAX, SFO and Seattle. They all have VFR transition corridors. Details about these are on supplemental or “ FLY” charts

Welcome to aviation.SE! This question has some related information.

@757toga Thanks!


text:
Does the FlightRadar24 pose a risk to aviation security?
As you may know, on the internet there is the FlightRadar24 website where you can see the flight information of planes worldwide in real time. Although this is wonderful for those who like aviation, does this type of system not pose any risk to aircraft safety? For example, terrorists may well plan an attack on a specific plane by knowing its information.

Reinforced cockpit doors, the related access protocols, and the post 9/11 assumption going forward that a hijacking will end in a crash, has pretty much reduced the hijacking risk to a much lower level, because the chance of success is way less.  There have been very few traditional hijackings since, with 1st world airlines at least. The airplanes are "hardened" just enough to not make it worth the trouble.

MH17 wouldn't have happened if the Russian-enabled group looked at FR24.

How does the position of an airplane aide in hijacking it? If you have a shoulder-launched anti-aircraft missile, you don't need a map of airplanes, you can just look up and find one.

"How does the position of an airplane aid in hijacking it?" Hypothetical (I hope) case: an organisation intent on blowing up a plane can see exactly where it's coming down and has a few minutes to find and remove the "black boxes" and perhaps other things of value before the emergency services or local military arrive.

@MarkMorganLloyd - How does capturing the black box after the destruction of the aircraft help a terrorist organization. The point of terrorism is to instill fear. Also, the transponder and ADS-b would stop working before the plane hit the ground. The terrorists could just find the FDR & CVR using their built in radio beacons. The only advantage gained would be to track the progress of their fellow highjackers in the air. But, the terrorists on the ground can do little to aid the ones in the air. Except to maybe alert an alternate landing strip to prepare for the aircraft’s arrival.

The short answer is yes it does. In reality, the amount of benefit Flight Radar gives to those that would exploit it for evil is negligible compared to the amount of good the technology provides.
The data for Flight Radar is drawn from ADS-b transceivers. It will only provide the present and historical position of the flight. The aircraft’s future position can only be speculated upon based on its flight plan. If a flight’s position is classified or of concern for national security, the ADS-b information will not be broadcast “in-the-clear”.
As far as terrorism, Flight Radar will not provide any more information than a good network of plane spotters would. After all, a cruising altitude of over 30,000 feet MSL would make all but the most advanced military equipment ineffective against a commercial aircraft. Below cruising altitude, line-of-sight weapons would make Flight Radar redundant to common binoculars and scopes. Terrorists aboard an aircraft would make Flight Radar a non-issue. Not to mention, the terrorists would probably try to turn off the transponder and ADS-b.
A similar argument was made for the use of GPS when it was first created. That is why the accuracy was purposefully limited by the government for the adoption by the civilian market.

You say "yes it does [pose a risk]", but then fail to give a reason why it is in fact a risk.

@zymhan - That is the point. Flight Radar does not pose any more of a risk than is already present otherwise. But, it can not be said that it poses absolutely no risk. Can any amount of intelligence or information pose a risk to aviation? Yes, it can. Yet, it is almost impossible to completely keep all information out of the hands of evil-doers. There is always a way to get the same information through other means.

GPS was not degraded for security, but to preserve a strategic advantage.

@bogl - You are absolutely right about GPS. But, I did not say that GPS was degraded for security reasons. Although, Denying the enemy a strategic advantage by limiting the information they can gather is, in itself, a security issue. Both GPS and now ADS-b still pose a risk that is beyond just a security risk. Both can provide an accurate way to relay the position of an asset or target to enemy command structure. Both can be used to track the movement of an asset or target. Both can also be used to guide munitions to that asset or target. But, so can a guy with a radio, map, & lensatic compass

The data FlightRadar uses is provided by volunteers using commercially available ADS-B receivers. All it does is save the terrorist a couple of bucks buying an ADS-B receiver.

That's a simplification, since my understanding is that FlightRadar24 aggregates the input of many ADS-B receivers to produce an overall report: "the sum is greater than the parts". I would, of course, be happy to be corrected.

The OP didn't really specify the threat model, unfortunately. A question about security really only makes sense with a detailed threat model of "securing what against whom". Major concerns with an airplane are hijacking and shooting it down. If you are hijacking the plane, you are already on it, and don't need FlightRadar. If you want to shoot it down, you only need to be able to receive the ADS-B when the plane is near you, to identify it, so one receiver should be enough. FlightAware uses the receiver aggregation for coverage, as far as I understand, not accuracy.

A more futuristic threat would be hacking the plane. Well, if you hack the plane, and get access to the flight computers, you don't need FlightAware either. So, in no case does it give you any advantage. And even if we assume that FlightAware were to somehow give an advantage, they are doing nothing that an attacker could not do themselves, given enough resources.

Plane movements are, for the most part, planned well in advance. Not only that, an airline actually has to publicize those times in order for customers to be able to book tickets. Want to find what times EasyJet is flying from London to Paris? Look on their website!
The only way terrorists could be helped by having position information is if they had a surface-to-air missile and they wanted to take out a specific aircraft. However in that case they would have to be somewhere on the flightpath for takeoff or landing in order to have a shot. And in that case they can simply listen to the radio between ATC and the various aircraft to find where they are.
So it doesn't add any extra risk that isn't already there.


text:
Can a commercial pilot rent an airplane and use it for hire?
Can the holder of a commercial pilot certificate rent an airplane (with a recent 100-hour inspection) from a fixed-base operator and use it to carry passengers for hire?
I believe the answer this is a "NO" since this pilot would be holding out by providing the airplane to the passenger, thus, this pilot would illegally be acting as a commercial operator / air carrier. Basically, the pilot and the airplane would be coming from the same source, even though the pilot doesn't actually own the plane and is renting it from somewhere else.
EDIT: Sorry, yes, I'm asking in regards to US regulations (FAA).

How would this differ from a charter operator using a leased aircraft (other than, say, 737 vs. Cessna 182)?

Why would the difference between renting the aircraft & owning it change the need for a Part 135 certificate?  (As well as the exceptions for when such a certificate isn't needed.)

Most FBOs that rent airplanes probably say that you cannot do this in the rental agreement...

or from a freelance instructor renting an aircraft to provide instruction in

It doesn’t matter whether the pilot rents, leases or owns the plane; if they are offering “aviation services” to the “general public”, then they are “holding out” and would need an Air Operator Certificate.

You're right.  In all but a few situations a commercial pilot is essentially the same as a private pilot(you pay your pre-rata share).  In a nutshell a commercial pilot can be paid to work for a carrier(part121/part135) or can be hired by an aircraft owner to fly the owners airplane on behalf of the owner and for the owners direct benefit(e.g. corporate pilot).  There are some special circumstances and exceptions the regulations cover, but in broad strokes the above is true.

The answer is no. A commercial pilot cant supply the airplane as well as their self. That classifies as a wet lease, which you would need an operator cert for. But a commercial pilot can have a friend or the customer rent the aircraft and be compensated. Known as a dry lease. It’s perfectly legal to pimp yourself you just cant pimp your airplane and yourself. Make sense?

Except the friend or customer would have to be checked out in the aircraft and act as PIC, so why would they pay someone else to fly it?


text:
Is it true that high-bypass engines are inherently inefficient at higher speeds?
Power consumption=Kinetic energy=$1/2mv^2$.
Thrust=momentum change=$mv$.
I keep being told that aircraft engines are most efficient when their exhaust velocity closely matches the freestream.

The second graph shows the propulsive efficiency vs exhaust/freestream velocity ratio
I can understand that higher bypass engines have greater intake losses than narrower ones. But the ratio of airspeed to exhaust speed being the defining factor, as opposed to the absolute difference?? I have only one way to justify this - as speed increases, mass flow rate through an engine of given cross sectional area also increases.
Research on propfans, and on supersonic through-flow turbofans calls into question the idea that lower bypass ALWAYS = higher high-speed efficiency. And the above equations don't help either.
What mistake am I making?

related: In what way are the Concorde's engines considered efficient?

the concorde engines had exceptional pressure ratio - and thus thermal efficiency - at speed. but this here is about propulsive efficiency.

Thrust is proportional to speed difference and area. But the propulsion efficiency of the engine drops as exhaust velocity increases due to losses in the exhaust stream. That means larger area and slower speed is inherently more efficient than small area and large speed.

@Jan I can understand that kinetic energy is not equal to thrust, but momentum is. And that is why higher BPR is beneficial. But why would we ever say that a lower BPR is more propulsive-efficient under certain circumstances?

The first graph you posted shows that propulsive efficiency of turbofan engines increases up to the point they no longer have enough exhaust velocity to produce thrust, contrary to your question. Also, what is the second graph supposed to show?

@JanHudec uh sorry I fotgot to label

Power consumption=Kinetic energy=$1/2mv^2$.
  Thrust=momentum change=$mv$.

This is only true if the engine is perfectly efficient internally and the air is still relative to the aircraft.
If the air is moving relative to the aircraft (and the engine is still perfectly efficient internally) then.
Power consumption=Kinetic energy=$\frac{1}{2}mv_e^2 - \frac{1}{2}mv_a^2$ = $\frac{1} {2}m((v_a + v_\Delta)^2 - v_a^2) = \frac{1} {2}m(2v_av_\Delta + v_\Delta^2) $
Thrust=momentum change=$m(v_e-v_a) = mv_\Delta$
Where $v_a$ is the ambient velocity (relative to the aircraft), $v_e$ is the exhaust velocity (relative to the aircraft) and $v_\Delta$ is the difference between the ambient velocity and the exhaust velocity ($v_e - v_a$).
If the exhaust velocity (relative to the aircraft) is smaller than the ambient velocity (relative to the aircraft) then you have negative thrust (aka drag).
If the exhaust velocity (relative to the aircraft) is much larger than the ambient velocity (relative to the aircraft) then most of the power is wasted (the $v_\Delta^2$ term is larger than the $2v_av_\Delta$ term)
If the exhaust velocity is only slightly higher than the ambient velocity then most of the power goes into useful thrust (the $2v_av_\Delta$ term is larger than the $v_\Delta^2$ term)
What this means if that if the engine were perfectly efficient internally then the advantages of a higher bypass ratio would drop off with speed, but there would still be a slight advantage to the engine with higher bypass ratio.
But that is a big if, in particular we assume that all the energy in the intake stream is captured and returned to the exhaust stream.

Lets consider what happens if we instead assume that the energy in the intake stream is lost.
Our power consumption equation becomes.
Power consumption=Kinetic energy=$\frac{1}{2}mv_e^2$ = $\frac{1} {2}m(v_a + v_\Delta)^2) = \frac{1} {2}m(v_a^2 + 2v_av_\Delta + v_\Delta^2) $
For a given thrust $m$ is proportional to $\frac{1}{v_\Delta}$ so our goal is to minimise. $\frac{1}{v_\Delta}(v_a^2 + 2v_av_\Delta + v_\Delta^2)$ = $\frac{v_a^2}{v_\Delta} + 2v_a + v_\Delta$ differentiating with respect to v_\Delta and setting to 0 gives $-\frac{v_a^2}{v_\Delta^2} + 1$ which (given that v_\Delta must be positive) gives $v_a = v_\Delta$

Having considered an optimistic and a pessimitic case we can conclude that the optimal exhaust velocity for efficency is greater than the ambient velocity (otherwise you would not produce thurst) but probably less than twice the ambient velocity.

Actually the energy of the intake stream is not only not lost, it increases the pressure ratio and therefore thermodynamic efficiency of the engine.

First, thrust = ma = mv/s
Next, what type of engine are you referring to?  Throw out the first graph, it's all about propulsive efficiency vs speed (props, fans, etc.), not about engine efficiency.
Considering exhaust only, the engine has to work to push out the exhaust.  This is why rockets have more thrust in space.  Also known as "backpressure". Efficiency will improve as exhaust velocity approaches freestream velocity (and will continue to improve if freestream velocity is greater).  A large widening exhaust pipe that exits to the free stream helps reduce backpressure.
But this is not good if you are trying to use your exhaust to make thrust.  However, with jets, the free stream also helps air intake as well.
As far as the title to your question, now take the first graph and look at props.  A "high bypass fan" has the same issue as a prop at higher speeds, it's own disk drag.  This has nothing to do with engine efficiency.


text:
Why did the L1011's direct lift control help make good landings?

In landing configuration the 1011 had what was called “Direct Lift Control” or DLC. That feature was actually pretty cool. Once the airplane was in landing configuration the spoilers would come up to a neutral position of 7 degrees. If you moved the yoke forward, asking for an increased rate of descent, the spoilers would come up to a maximum of 14 deg but the pitch would not change. If you pulled the yoke back the spoilers would move toward 0 deg or stowed, thereby increasing lift and slowing descent. The DLC worked really well and made for super stable approaches with rock solid pitch attitude and rewarded you with “greaser” landings more often than other big jets I have flown.

Source
Why does altering sink rate without changing pitch make for a super stable approach and "greaser" landing? The explanation I can think of is that moving spoilers is faster than pitching, allowing faster reaction times to errors.
EDIT: B787 has something similar

@ymb1 click on the link and wait a bit. The link should take you directly to a comment.

The 737 MAX also has this, in order to assist with landings in the event of an elevator jam.

Direct lift control reduces the need to change pitch attitude to change total lift to change descent rate and therefore fine tune glideslope.
Changing pitch attitude introduces inertial forces in the pitch axis; you have to get this long body with a lot of inertial mass rotating, then you have to stop it.  You are doing this with elevator, and there is a lag between input and result when starting and stopping a movement.
On a big airplane like the 1011, with a lot of mass in the fuselage (tail mounted engine and compensating fuselage mass forward), trying to find tune descent rate with small pitch attitude changes can become a game of chasing input/result because of all the inertial mass and lag.
Using spoilers in that way, and seamlessly integrating it into the  elevator control system in landing configuration, allows this sort of fine tuning of descent rate to be done without having to get body moving about the pitch axis, allowing extremely precise and near instantaneous adjustments to glide slope that would normally be impossible with all that mass to start, and stop, rotating.
The characteristic would feel a bit odd at first when flying the 1011, when you make a pitch input and the attitude doesn't change, but you would get used to it pretty quickly and the instant sink rate response would be a great feature in a big airplane like that.

John's fine answer is correct, but there is a bit more to it:
When increasing lift by pulling on the yoke, in most airplanes the elevator moves trailing edge up, decreasing lift initially. To look at the process in detail:

The yoke is pulled, the elevator moves trailing edge up
Downforce on the horizontal tail is increased without any lift change on the wing, so total lift is diminished. The sink rate increases slightly.
The lift imbalance adds a pitching moment, so the nose of the airplane rises.
Now the angle of attack on the wing increases with the pitch angle and lift increases. Only now will the desired reduction of the descent rate kick in.

This effect is especially pronounced in flying wings, but it also is noticeable in regular airplanes. Now compare that with partially deflected spoilers:

The yoke is pulled, the spoilers close.
Lift increases without a change in the elevator angle or the angle of attack. The desired reduction of the descent rate is immediate.

The effectiveness of spoilers is limited, so this technique will not allow you to fly aerobatic maneuvers, but for landing it is perfect. Drag is increased, so the engines will have to produce a bit more thrust than with spoilers fully closed, giving you more immediate engine response, too.


text:
How long to receive FAA wings lapel pin?
Did anyone request FAA wings lapel pin? If yes, how much time do they take to ship and receive? Do you know if I requested all three phases at once will I get three or just one pin?

Related: Is a physical lapel pin awarded for FAA Safety Wings?

From deleted comments somewhere else on this website, it seems that it can take several months before they arrive.

I requested all three FAAST WING lapel pins around, but not at, the same time. I think the first two pins were at the same time. It took over three months to receive the first one. When I enquired about the others, I was told that Avemco Insurance (the sponsor of the awards program) orders them in bulk. They ship out the awards only when they have them in stock. So, your experience my vary depending on Avemco’s ordering cycle.


text:
Course of other planes approaching the airport
I flew from MUC to TXL recently and made a weird observation. I was sitting in the port side of the plane and -just a few minutes before landing- saw a red light at approximately our height just before our wing tip. "Ok, I thought, that's odd. That other plane appears to be flying in the opposite direction but it cannot have just started, so maybe a holding pattern or something". While I was pondering that our plane switched on the landing (?) lights. And to my irritation that other plane did as well. "If they are about to land, should they not be behind or in front of us?", I thought.
But what irritated me the most was that the relative position (i.e., the bearing) of that other plane did not change at all. I've learned in boat school that another object with a constant relative bearing is probably on a collision course. It could also be on a parallel course with the same speed, but we were descending towards the airport - TXL does not have parallel landing strips AFAIK. I have no idea how far away that other plane was.
So here are my questions:

Is my assessment of the situation correct or do I miss something?
Is there a point during a landing approach were planes can be on a collision course (because one or both are to make course changes), I always thought that planes would be arranged in some long line twoards the airport, much like a highway.
When is it a good idea to mention such an observation to the crew? How would one do it without looking like a busybody?

edit: Thanks to the great comment about flightradar (I did not know about the playback), I could sort out what probably happened.

There was indeed a flight on a collision course. A plane from STR on its way to TXL came from the south. We came (roughly) from south-southeast. Our flight paths would have crossed somewhere north of TXL. We turned right into the queue first, the other plane did the same, but slightly behind.
A third plane heading southwest crossed roughly (from my perspective) the line between the two planes.

So I must have seen the third plane first and then the second plane. We were indeed on a collision course, but with a rather large distance and with both planes scheduled for right turns.

You can go to flightradar24 and see exactly what aircraft you saw and its relationship to your plane

It almost sounds like you're describing a reflection of the aircraft you were in, but I don't know what conditions would allow that. Did you actually see the other aircraft, or just its lights? And were you in clouds?


text:
Why have propeller engines never been mounted on the tail in production transport aircraft?
Recently, Embraer teased this concept.

Unlike jet engines, I have never heard of a production airliner or executive aircraft that mounted propellers on the tail like this.
From the source:

the new Embraer turboprop will have rear fuselage-mounted engines. This, it says, will provide a quiet cabin and allow jetbridge compatibility.

from the comments section

Well the nice thing about rear mounted engines, turbofan or turboprop is that it free's designers to be much more creative with wing design. The classic example of this is the VC-10, although a commercial flop with rear mounted quad engines had great aerodynamics

It will certainly be more efficient as it keeps the wing clean, not to mention whatever powerplant and propeller combination they use will be cutting edge

Why were these advantages not considered relevant earlier?

Why do you think that?  Here's an example of tail-mounted propellor, the Lear Fan: https://en.wikipedia.org/wiki/LearAvia_Lear_Fan  There are fairly obvious technical problems that need to be solved, like weight & balance, and protection against prop strikes.

There are even examples of a tractor propeller mounted on the tail: See here. And here

Incredibly poorly-researched question. Multiple production aircraft have had tail-mounted props as above comments demonstrate.

@IanKemp research be like "airplane with rear mounted propeller" "airplame with rear mounted propellers" "airplane with tail mounted propellers" "airplane with tail mounted propellers like in bizjets" "airplane with tail mounted propellers like in business jets" "airplane with tail mounted propellers instead of wing mounted propellers".......

"aircraft with tail mounted propeller engines" then images, gets me several. Yes, it includes wing-mounted pushers, but there are some tail mounted planes shown, which immediately invalidates your title question: Why have ... never been ...? Well, there HAVE been, so it's a fallacy.

@CGCampbell may I assume that my body is still valid?

@mins yes, I edited the title some time after that comment was made. Before, there was no mention of transports.

@Ian Kemp: "Incredibly poorly-researched question", "incredibly" may be a bit too much, specially since the examples posted prior to your comment are for single-engine aircraft, not really comparable with the aircraft in the question.

@jamesqf, he did say "production airliner" in the question body (even before adding it to the title), and the Wikipedia entry for the Lear Fan says that it never entered production. And when I google "aircraft with tail mounted propeller engines" I get push-pullers, ekranoplans and conceptual stuff. Search results are personalized, so somebody who does not regularly research aviation stuff might get much more underwhelming results than somebody with a specific search history.

Hmmm, I'd think as the plane pivots on take-off, rear props would oblige a higher mounting then mid-plane ones.

There are two really big issues with tail mounted powerplants with propellers; weight distribution and Foreign Object Damage.
Tail mounted engines move the empty C of G aft.  This forces you to put the disposable load with a forward bias to compensate, hence the long forward fuselage necessary to put the loaded C of G in the required range.  Of course, when passengers get off, the C of G moves aft, and to keep the tail from tipping back while the plane is unloaded, you have to move the gear farther aft than optimal for takeoff.  The elevator surface has to be larger than it might normally be in order to lever the nose up on take off with the fulcrum of the tire contact point farther aft than it really needs to be. With so much fuselage forward of the loaded C of G, you might have to use larger stabilizing surfaces to have adequate tail volume for good yaw and pitch stability.
Jets with tail mounted engines live with these problems to gain the huge advantage they provide, the ability to have the fuselage close to the ground to make it easy to get in sitting on a ramp, an important feature for corporate airplanes.  For an airliner that uses jet bridges for boarding, this feature isn't important.
Then there is the brutal environment for the poor propellers.  Every bit of rain, slush, stones, rubber fragments, and whatever other garbage is on the runway will be going through the propellers, and they will get dinged up and eroded pretty fast (anyone that's owned a seaplane understands the mess that large volumes of water makes of propeller blades).
For an airplane with propellers, back at the tail is about the worst place you can put them if propeller service life is important.

Your second-last paragraph would appear to be more correct for wing-mounted props, which are closer to the ground due to wing position and thus would be more susceptible to FOD than tail-mounted ones. In fact, due to being closer to the fuselage than wing-mounted props, a tail mount might well decrease FOD. In the case of freezing conditions though, definitely the tail-mounted props would be eating slush and ice, while wing-mounted props would not.

Further, the question is specifically about tail-mounted prop engines, but your 2nd and 3rd paragraphs address tail-mounted engines in general.

Apart from CoG position and FOD, the elephant in the room is the risk of a prop strike. A propeller typically has a significantly bigger diameter than a turbofan of the same thrust/power, so you can't just put turboprops in the same spot where tail-mounted turbofans go. And while a tailstrike on takeoff is undesirable in a jet, having your sole means of propulsion hit the ground on rotation (thus way past V1) would make for a really bad day at work.

@TooTea As the aircraft rotates for takeoff the tail and props will indeed get closer to the ground, but the angle of the props will also change from being perpendicular to the ground to being more parallel with it. So depending on the prop diameter and how far forward they are set, it's entirely possible that there's no possibility of prop contact with the ground even in a tailstrike scenario (and in fact I would hope/expect this is how Embraer has implemented this concept).

@IanKemp Yes, the Embraer pic seems to show the pylons angled up (with a significant dihedral), presumably for either ground clearance or wing wake avoidance.

@IanKemp No the wing mounted prop close to the ground only pulls things into it while stationary at high power settings.  This is a big problem for piston engine planes that have to do runups. Turboprops don't have to do that. Tail mounted propeller gets every bit of crap thrown up by the tires.  For the best illustration, take it to the extreme; you can run a King Air off a gravel strip without too much propeller grief, but put the propellers back there, and fuggeddaboudit. Plus if the plane operates in rain  a lot, the erosion problems  will be significant. Again, water thrown up by tires.

The weight distribution aspect may change with lightweight electric motors and heavy batteries.

@user_1818839 Putting (some of) the batteries in the nose might help. As for weight distribution in general, there are plenty of military jets with tail-ish mounted engines (inside the fuselage of course) which have a big lump of concrete built into the nose to put the CG where it needs to be!

@alephzero that leads to another problem with putting things at the ends.  You have strong polar inertial forces which are generally a bad thing from a handling and behaviour perspective, e.g. spin recovery, etc.

For a center pusher-configuration prop (like the XP-55), since the wings are now in front of the engine, does that also create any design considerations around the formation of turbulent flow between the trailing edge of the wing and the propeller?

"For an airplane with propellers, back at the tail is about the worst place you can put them if propeller service life is important" - this is a great answer for why this configuration is so rare. But it also answers why they do an exception in the current case: as a luxury aircraft, it seems they considered that the reduced noise and therefore better comfort is worth the increased maintenance costs.

You can make a case that this was the original way to build airplanes as the Wright Flyer had a forward elevator and rear-ish mounted props extending off the back of the wings. Other than that there have been many designs to incorporate rear propellors.

The Bede BD-5 has a rear prop. Burt rutan built various designs with rear mounted piston engines/props. The Piaggio P.180 Avanti is pretty close to what you describe. The Otto aviation 500 has a rear prop but is only now being tested. The Dornier Do 335 had both a front and back prop

Fairly unsuccessful, but also the Beechcraft Starship.

@JerryCoffin that was a wing mounter

As stated in comments and Dave's answer, it has been done, but I'm guessing you were thinking about mass produced (successful) commercial passenger planes. Here are a couple of reasons:
The propellers on a passenger plane need to be quite big, so they would, for clearance reasons, need quite long and thus heavy pylons. Weight in the tail of a plane is generally not a good thing. The more modern fan-like propellers are smaller, and this somewhat mitigates the pylon problem.
Mounting propeller engines on wings is easy, as clearance from fuselage is not a problem, no extra structures are needed and the engine nacelle/fairing often provides a convinient space for landing gears. One advantage also is, that you have "blown flaps", but I think you are correct in that having undisturbed airflow over the wing would be beneficial.

https://en.wikipedia.org/wiki/Propfan#1970s%E2%80%931980s
That Wikipedia page has a good analysis of propfans a.k.a. unducted fans.  See the MD80 testbed and reference to various designs (e.g. Boeing 7J7 slated to be the 737 replacement) with rear-mounted propfans.  While not coming to fruition evidently it was considered to be a possible alternative to underwing engines (ducted or unducted).

Since mounting the engines up high makes them more difficult for mechanics to service and inspect, you would want to have a good reason for putting them there.  It sounds like Embraer's reason is sound (and that fact that their entire line and assembly and design process is based on aft-mounted engines).
I cannot cite any evidence for this hearsay claim, but... years ago I read the comment from a former Boeing employee who said that when they inspected the Boeing 727-100 that was used in 1986 as a test bed for GE's UDF (un-ducted fan), they found that the vibration from the UDF had damaged the structure of the aircraft.
When comparing wing-mounted prop engines to aft fuselage-mounted engines it occurs to me that you have to run fuel lines from the wing tanks to the aft fuselage (weight) and, in the event of a prop failure/separation there is a higher probability of the prop penetrating the fuselage, and possibly even the fuel line.
Not to mention the (remote) possibility of a failed prop destroying the rudder, tail, elevators or horizontal stabilizer.
Of course catastrophic prop failures are extremely rare (or less), and (to my knowledge) have never once been a problem for aircraft like the Beech Starship or Piaggio Avanti.

Fuel efficiency is very important for transport bottom line profit, which is why wing mounts are generally seen.  A number of ex-passenger liners such as the Boeing 767 are re-purposed as cargo converts.
The Embraer concept moves them back for cabin noise mitigation, but rear mounts give back a little efficiency because a father rearward CG shortens the tail torque level arm, therefor requiring larger tail volume.
For cargo carriers, the best place for the turboprop pylons would be near the nose of the aircraft (if the pilots could stand the noise), a bit like a hammerhead shark, or on the wings.  The nose mount would again require a larger horizontal stabilizer (because of increased area ahead of CG).  Interestingly, the rear mounts do increase stability a bit. (Aerodynamic effects cancel CG effects for tail sizing).  One can see why engine nacelles have been generally mounted on the wings, closer to CG, for a long time.
Also of note, the Embraer design features a biplane horizontal stabilizer, why not mid-mount a combined pylon/stabilizer for better aerodynamic efficiency. This would also alleviate debris damage and rotation issues with the props.
Over many flights, fuel cost is the deciding factor in design for cargo aircraft.

Indeed, but for a luxury aircraft the added comfort due to the reduced noise might make it worth the increased fuel and maintenance costs.

Tail mounts are viable, and leave a super clean wing.  There is the issue of damage from ice shedding that would make me wonder if the current turboprop design will work.  Avanti seems to have had some sucess with 2 pushers on the wings.  Wing pullers have a long history of service.


text:
Why are there so few high-wing jetliners?
So far, we have only seen a few civilian jet transports with high wings. All of them BAe 146, Do 328JET, seem to be regional jets. Yet military jet transports are almost completely high-wing. Why aren't we seeing high-wing large jetliners?
Some advantages of high wings:

shorter landing gear
easier access to the fuselage
a better location for the center-of-thrust.
better stability
better ground clearance for upcoming high-bypass turbofans. Lack thereof caused the design changes that ended in 737 MAX crashes
Increased flap efficiency
better able to take advantage of strut-bracing, if any.

Some disadvantages:

less efficient location of landing gear. (probably not mounted on the wings)
more likely to tailstrike
passengers may mock
belly landings could be tricky
water landings may be even more tricky

I don't know how these pros and cons add up though. Can someone give some more info?

The 737MAX would had become unstable regardless of wing position. The forward shift of larger engines is the problem, and the solution would be a larger tail. If only Boeing "management" would accept that simple truth.

Also, the MAX fiasco has nothing to do with center of thrust. It's all about aerodynamics, where the new nacelles generate lift at high enough AoA, causing an unwanted pitch-up moment.

The larger engines couldn't be hung below the wing, so they had to be moved up and forward. Then, what you say happened,

@PeterKämpf if MAX was a high wing design, there might not have been the need to place the bigger engine more forward than the older and smaller ones. This would all depend on ldg gear design, of course...

"passengers may mock" -- hi, Larious.

@Jpe61: You're right.

Why would a high wing mean shorter landing gear?  I'd think just the opposite.

If gear is placed on the bottom sides of the fuselage (like C-17 for instance) then yes, they would be shorter. There is no reason to attach ldg gear to wings on a high wing jet.

"passengers may mock"?? I always prefer high-wing planes, as a passenger or a pilot. Better view, for one. (Should be in the list of advantages, for sure).

@Jpe61: Sure there is, to have the gear a reasonable distance apart for stability.  Compare for instance the wing-mounted gear of a low-wing Piper Cherokee to the fuselage-mounted gear of a Cessna 172, which sticks out at an angle to either side.

@jamesqf I'd argue the C-17 is in substantial need of stability, especially with it's outboard engines.

@jamesqf "...a high wing jet..."

@Jpe61: Same principle at work.  I just don't happen to own or fly any jets, so can't speak from experience about them, or FRM really call any particular ones to mind.

I got duped and one of the questions I'm linked to is also duped haha. I suppose a difference between those questions and mine may be that mine focuses on large jetliners, while they're more general (turboprops, pistons).

Large commercial aircraft have low wings

to easily stow away their long landing gears in the wing root,
to hide the wing spar carry-through below the passenger deck,
and to improve the accessibility of wing-mounted engines.

Long gears make it possible to stretch the fuselage and still be able to rotate during take-off. Stretching makes it possible to tailor one basic design to a wide range of sizes, lowering the development cost of a single aircraft.
Pressurized fuselages need to have their passenger floor a bit below the center of the fuselage tube, leaving ample space below for cargo, systems and structure. Placing the wing high would produce a hump on the fuselage, which adds wave drag at the transsonic cruise speed of typical airliners.
The low position of the wing-mounted engines makes them much better accessible for maintenance; a major reason this configuration was selected in the early jet transports. Civilian freighters use mostly converted passenger planes, and their small number precludes custom designs. Initial concerns about foreign object damage (which resulted in a number of early jetliners with tail-mounted engines) proved to be unfounded.
Large high-wing airplanes with their low fuselage position are easier to load and unload, at the price that the fuselage taper has to start shortly aft of the landing gear, so no stretching is possible. The military doesn't mind and prefers the high-wing variety. Also, the high wing reduces the risk of engine damage by foreign objects on improvised runways. That Lockheed likes to stretch their transports (C-130, C-141) anyway is the exception that proves the rule.
Turboprop-powered civilian airplanes may have a high wing in order to provide more space for the propellers. Here, the fuselage-mounted landing gear can be made short and light (which is how the Do-328 JET evolved). Still, both versions exist. And the ones where the engineers did not know how to attach a jet close to the wing*.
Aerodynamically a mid-wing position would be best. This is used when the payload is compact and needs little space, or is hung externally. In bombers, in other words. Civilian transports will always either have a low or a high wing since nobody wants their passengers to climb across a wingspar carry-through on their way to their seat. At least since 1933.

*  To be fair to the Avro engineers: The high wing position gives that airplane a very high maximum lift coefficient without requiring slats.

The 328 was an odd kind of dead end design, mostly because of its straight wing, and it could only cruise at m .66, up to FL350.  That cruise speed made it too slow for the FLs above 300 in busy airspace, so ATC would put them in the high 2s all the time where the fuel burn was sub optimal.

Dash 8 have been stretched repeatedly; but its gears extend from the engine nacelles rather than the belly.

@JohnK: As a turboprop (as intended) it was excellent. Only a desperate attempt to make it into a jet turned it into that odd design. Dornier by that time had a 728 on the drawing board with swept wings and all, but they wisely decided not to enter that crowded market.

Peter, is it not also true that a high-wing approach moves both the wing and the engine nacelles up away from the ground in military transports, thereby minimizing foreign object damage when operating out of unimproved airstrips?

@nielsnielsen yes, that's another advantage of the high wing.

@nielsnielsen: You are right. I focused on jetliners here as the question suggested and only gave the most important reason for high wings in military transporters. And in some turboprops. And in one British oddball.

The really big show stopper in the high wing/low wing issue for airliners is bullet point #3 in Peter's answer; the need to place the wing box for the high wing mostly outside the circumference of the fuselage, so overall frontal area is higher and you have this big projection above the fuselage.  This is driven by the need for headroom in the cabin as shown below:

The DeHavilland Dash 7 had insufficient headroom in the center aisle under the wing because they were trying to get the wing box as low as possible, and it had to incorporate a dip in the aisle below the surrounding floor, where it ran under the floor, to gain an extra inch or so of headroom.
Another minor factor is the wing to fuse attachment structure has to accommodate tension loads since the load is hanging, whereas with the low wing, the fuselage just sits on the wing box with much simpler structural connections, so the high wing's arrangement is a bit heavier.

High wing attachment is indeed heavier, but mainly because it needs reinforcement between the wing [spar] and the landing gear (if it is attached to the fuselage). Otherwise, tension loads for thin/slender structures like airplane's are actually better than compression.

Yes but you need complex fittings and hockey stick frames that fork at the top to distribute loads with the more narrow interface at the top. Much more elaborate connection.  In the 70s  I did wing marry-up on Dh -7s.  A low wing just has maybe 6 links or lugs joining the wing box to the fuse.


text:
Any idea what this Ampaire plane is?

(Source)
This is certainly not the Ampaire 337 or Electric EEL, which is a converted Cessna 337. It looks like a boundary layer ingestor. Being from Ampaire, it probably is electric or hybrid.
I've tried to find more information about this craft, but failed so far, even from their own website (including the linked page). Google seems to think it's a Learjet. I don't think I've been able to find much, if any other photos of this plane.
The foreground seems to indicate this was a real photo taken from another plane, as you can see window sills, details generally absent in renderings (and many show-off photos). I must admit though, that's not the safest flying around.
Even if it's a rendering, it would probably be a concept. I can't even name the plane yet.
It's beautiful.

This does not appear to be a real picture. It looks computer generated.

@DeanF. Are you sure? Notice the window sill in the foreground. I don't usually see such things in CGI

I am not sure at all. But it could be a screen capture from simulator software. Do you have a link where you found this?

@DeanF. yes, I had one all along.

The only other places I'm finding this photo are news articles which all mention the EEL - the wrong plane. Not a clue about this airplane.

I am unable to find this specific picture at the link you provided. There are other still pictures under “Media”. Some of them are real and some of them are artist renditions. Can you update your link to point to the actual picture?

@DeanF. Uh, sorry, wrong page. I'll fix it.

@DeanF. should work now. You'll have to scroll down a bit.

Definitely an artist rendering. The chase plane window sill is placed for realism.

@Abdullah: "Are you sure? Notice the window sill in the foreground. I don't usually see such things in CGI" – Why does it matter whether the window sill in the foreground, or for that matter, the landscape and sky in the background are real or CGI? In fact, I wouldn't be surprised if they were real! Why go to the trouble of rendering something when you can find thousands of photographs on the web? But the plane in the center of the picture is definitely rendered. The skin just looks "off".

@JörgWMittag yes, this has been confirmed below

The amateurish wing-fuselage integration gives this away as one of the many scams with electric aircraft which circulate today. Every naive fellow thinks after Tesla there must follow electric airliners. Do the numbers and see for yourself how realistic this is.

@PeterKämpf you mean it looks like the wings are glued on?

@Abdullah: Yes, and there is litte structure to support asymmetrical loads. With lowered flaps there is a huge hole, lowering their effectiveness. Next, the thing is unstable without thrust. This is a problem in a descent and when batteries are empty. No sane FAA official would sign off on this. Again, looks like a scam.

@PeterKämpf I suppose the stability might be provided by airflow around and through the engine nacelle?

@PeterKämpf Ampaire AFAIK is currently converting existing planes (twin otters etc) to electric.

It's called the Ampaire Tailwind, and as this article explains, it only exists in scale models and renderings at the moment.

this sounds like a stupid kind of thing for me to miss